makeman: add minimal support for group options

Ignore OPT_* values in showconfig out in exising code paths and add
a new path to include descriptions for each. For now, hardcode the
description contents rather than attempting to generate it. This runs
the risk of docs getting out of date, limits the amount of new shell
code added today while a lua rewrite is nearly ready to land.

This change requires a followup commit to enable OPT_* values in
"make showconfig" in order to actually find group options.

Reviewed by: emaste
Differential Revision: https://reviews.freebsd.org/D41681